The word 'prion'means 'infectious protein', a protein which can transmit a disease or trait without the necessity for an accompanying nucleic acid. The prion concept has its origins in studies of the mammalian transmissible spongiform encephalopathies (TSEs), a group of uniformly fatal diseases whose underlying cause appears to be the formation of amyloid composed of the PrP protein. Scrapie of sheep is a TSE that is readily transmitted (by injection) to other sheep, but to goats only after a long incubation period. Subsequent goat to goat transmissions show a much shorter delay. This is the original 'species barrier', and more dramatic versions are well known. Hamster scrapie is not transmissible to mice at all, and mouse scrapie only with difficulty to hamsters. In 1994, we described two prions of the yeast Saccharomyces cerevisiae, called URE3 and PSI, based on a self-propagating inactivation of the Ure2 and Sup35 proteins, respectively (1). Ure2p is a regulator of nitrogen catabolism and URE3 strains show a derepression of the genes normally repressed by this protein. Sup35p is a subunit of the translation termination factor, and PSI+ strains show elevated readthrough of translation termination codons, a phenotype similar to that of a sup35 mutant. In each case, prion formation is the conversion of most of the protein to an amyloid form, a filamentous beta-sheet rich form in which the protein is not available to carry out its normal function (reviewed in ref. 2). We have shown that the URE3 and PSI+ prions are diseases of yeast, detrimental to their survival or propagation such that neither prion is found among 70 wild strains in spite of both prions being infectious (3). In contrast, all the viral or plasmid nucleic acid infectious elements known are found in some fraction of the wild strains (3). We have shown that the variation in evolution of the Ure2p prion domain is much more rapid than in the C-terminal domain (4). This could have been due to lack of functional constraints on the prion domain, but we have shown that it is indeed important for full function of Ure2p (5). We suggest that variation of the Ure2p prion domain is actually selected in order to protect cells from acquiring a URE3 infection from other strains. This is analogous to the proposal by Collinge that the polymorphism at residue 129 (Met/Val) in the human PrP is (or at least was) maintained in the human population to minimize the effects of cannibalism (6). To test this hypothesis, we have examined the Ure2p's from various cross-breeding species of the genus Saccharomyces. We find that here too, the variability of the prion domain far exceeds that of the C-terminal domain. Moreover, there is a substantial 'species barrier'for transmission of URE3 from cells with Ure2p from one species to cells expressing Ure2p from another species (H. Edskes, L. McCann, R. Wickner, in preparation). It has been suggested that the presence of the prion domains that are dispensable for the usual function of the protein implies that the prion formation itself is being preserved in evolution. However, we have shown that the prion domain of Ure2p is in fact necessary for the full function of the protein in nitrogen regulation (7). Moreover, we find that the Ure2p's of some species are unable to undergo the prion change (8). Certainly in those cases, one cannot argue that the N-terminal domain is maintained for the purpose of enabling prion formation. Prion 'variants'are different prion isolates in which the prion protein is identical in sequence and the host is identical, but the phenotype produced and/or the stability or other heritable characteristic of the prion is different. Prion variants are believed to result from differences in the structure of the corresponding amyloid fibers. We find that different prion variants show different species barriers (8). This result is reminiscent of the reduced species barrier shown by the BSE strain of TSE compared to other variants, even propagated in identical mice. Understanding the relation of prion variants, species barrier and amyloid structure is a key aim of this and other projects in which we are engaged. Thus we are preparing various prion-forming and non-prion forming Ure2 proteins and are examining their amyloid-forming abilities and amyloid structures (A. Engel, H. K. Edskes, and R. Wickner). We are similarly studying the Sup35p's from various species to determine whether they can form prions and to examine the species barriers among them. 1. Wickner, R. B. URE3 as an altered URE2 protein: evidence for a prion analog in S. cerevisiae. Science 264, 566 - 569 (1994). 2. Wickner, R.B., Edskes, H.E., Shewmaker, F., and Nakayashiki, T. (2007). Prions of fungi: inherited structures and biological roles. Nat. Microbiol. Rev. 5, 611-618. 3. Nakayashiki, T., Kurtzman, C.P., Edskes, H.K., and Wickner, R.B. (2005). Yeast prions URE3 and PSI+ are diseases. Proc Natl Acad Sci U S A 102, 10575-10580. 4. Edskes, H.K., and Wickner, R.B. (2002). Conservation of a portion of the S. cerevisiae Ure2p prion domain that interacts with the full - length protein. Proc. Natl. Acad. Sci. U. S. A. 99 (Suppl. 4), 16384-16391. 5. Shewmaker, F., Mull, L., Nakayashiki, T., Masison, D.C., and Wickner, R.B. (2007). Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae. Genetcs 176, 1557 - 1565. 6. Collinge, J. (1999). Variant Creutzfeldt-Jakob disease. Lancet 354, 317-323. 7. Shewmaker, F., Mull, L., Nakayashiki, T., Masison, D.C., and Wickner, R.B. (2007). Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae. Genetcs 176, 1557 - 1565. 8. Edskes, H. K., McCann, L. M., Hebert, A. M., and Wickner, R. B. (2009) Prion variants and species barriers among Saccharomyces Ure2 proteins. Genetics 181, 1159 - 1167.